The present invention relates to precompensation of magnetic transitions written to a medium such as a disk, and more particularly to an apparatus and method for adjusting the shape of the write current waveform generated by the write driver to compensate for timing irregularities occurring in a high data rate magnetic recording system.
In magnetic data recording systems such as disk drives, data are recorded on the medium (i.e., magnetic disk) as a series of magnetic field transitions. In many typical systems, a magnetic transition which occurs at or near an integer multiple of the bit cell time represents a binary xe2x80x9c1,xe2x80x9d while the lack of a magnetic transition at that same time represents a binary xe2x80x9c0.xe2x80x9d A magnetic field is typically created by passing a current through a write head adjacent to the medium, creating a xe2x80x9cwrite bubblexe2x80x9d which defines a region in which the magnetic field is sufficiently strong to be magnetically recorded on the medium. Magnetic transitions are created by reversing the direction of current flowing through the write head.
The process of reversing the direction of current flowing through the write head requires a finite amount of time, often referred to as the xe2x80x9crise timexe2x80x9d of the write driver employed by the head. The xe2x80x9cwrite bubblexe2x80x9d created by the head field correspondingly contracts as the current is reduced to zero and expands as the current in the opposite direction increases to its steady-state value. The time required for the write bubble to expand to near its steady-state dimensions is referred to as the xe2x80x9cflux rise timexe2x80x9d of the head.
In high performance disk drive systems, the data recording rate can be high enough that the write bubble is unable to fully expand to its steady-state dimensions when the data to be recorded requires two or more consecutive magnetic transitions. The premature collapse of the write bubble (compared to other write bubbles which are allowed to develop fully) results in displacement of the location of the magnetic transition (which is defined by the location of the trailing edge of the write bubble when the write bubble expansion velocity is equal to the linear velocity of the media) from the ideal location of the transition edge by some non-linear amount. This phenomenon is known as a finite rise time xe2x80x9cnon-linear transition shiftxe2x80x9d (NLTS) in the magnetic transition pattern. A NLTS in the magnetic transition pattern of a disk drive system may also be caused by interactions between the magnetic field forming the present transition and the demagnetization fields of previous transitions and by timing shifts resulting from write current directions changes that begin from different relative positions on the settling portion of the write current waveform from a previous transition. These transition shifts can potentially cause errors in reading data from the disk, effectively limiting the data recording rate of the disk drive to a level at which the magnitude and frequency of occurrence of transition shifts are sufficiently low to ensure accurate data recovery from the disk.
In the prior art, transition shifts have been accounted for by a process known as precompensation. Precompensation systems examine a pattern of data bits to determine whether the pattern causes a NLTS to occur, and adjust the timing of magnetic transitions to compensate for the transition shifts. Precompensation may be accomplished by examining previous data bit patterns, known as xe2x80x9clook behindxe2x80x9d precompensation, or by examining future data bit patterns to be recorded, known as xe2x80x9clook aheadxe2x80x9d precompensation and described in detail in U.S. application Ser. No. 09/845,365 filed Apr. 30, 2001 for xe2x80x9cLook Ahead Write Precompensationxe2x80x9d by C. Elliott, J. Leighton, D. Galaba, T. Adams and S. Doherty.
Precompensation systems such as those described above typically employ programmable delays to adjust the timing of recorded magnetic transitions. However, the use of delays to adjust transition timing reduces the time between transitions, by as much as 25% in some embodiments. The reduction in bit cell time forces the write driver to achieve steady state in a shorter amount of time, which places a heavy strain on the write driver. It would be an improvement in the art to provide a precompensation system for adjusting the timing of magnetic transitions without employing delays, such as by modifying the write current waveform in a controlled manner. Such a system is the subject of the present invention.
The present invention is a disk drive writer precompensation system that modulates the write current waveform based on a pattern of data bits magnetically recorded on a medium in order to adjust the timing of magnetic transitions recorded on the medium.